Various systems have been proposed for sensing the interior volume of freight cars and trailers for cargo, and for transmitting information gleaned from the sensors to a remote site such as an asset management center. See, for example, the U.S. Pat. No. 6,437,702 to Ragland et al., the U.S. Pat. No. 6,919,803 to Breed and the U.S. Patent Application Publication No. 2004/0140886 to Cleveland et al. The sensors proposed in these patent documents range from simple ultrasonic transceivers to laser radar sensors. But in practice, sophisticated approaches using laser radar sensors are cost prohibitive, and economical approaches using ultrasonic and other fixed beam sensors fail to provide sufficiently detailed and reliable information. Accordingly, what is needed is an improved cargo sensing apparatus that provides a sufficient level of detail in a cost effective manner.